Bone screw and fastening tool for same

ABSTRACT

The invention relates to a bone screw and a tool for fixing same. A bone screw has a screw element ( 1 ), comprising a screw shank ( 2 ) and a screw head ( 3 ), a cylindrical receiver part ( 5 ) with a bore coaxial to the cylindrical axis coming from one end ( 8 ) of the cylinder, an opening ( 6 ) extending crosswise to the cylindrical axis for receiving a rod ( 16 ) to be connected to the bone screw, an inner thread ( 10 ) at one end ( 8 ) and a screw ( 13 ) cooperating with it and also an outer thread ( 11 ) at one end ( 8 ) and a nut ( 14 ) cooperating with it. Both threads ( 10, 11 ) are constructed as running in opposite directions. A fastening tool ( 21, 21′ ) acts as tool for fixing a bone screw, with a central shank ( 23 ), which is constructed at one end for engagement with a screw or nut and at its other end has a first handle ( 22 ), a sleeve-shaped shank ( 24 ), surrounding the central shank ( 23 ), which is constructed at one end for engagement with a screw or nut and at its other end has a second handle ( 25 ), both shanks ( 23, 24 ) being rotatable with respect to one another.

[0001] The invention relates to a bone screw according to the preambleof patent claim 1 or patent claim 2 and also an associated fasteningtool for fixing the bone screw.

[0002] A bone screw is known, for example, from EP 0 483 242 B1. A bonescrew according to the preamble of patent claim 1 is further known fromEP 0 614 649 A1. With these bone screws high torques are applied whenthe screws are driven home and also for fixing the screw element and forfixing the rod. As the screws have to be finally tightened during theoperation by the person performing the operation—after inserting therod—there is a danger of damaging the vertebra concerned. If the screwsare not sufficiently finally tightened, there is a danger that thescrews/rod connections will come loose and the bone screw will lose itsfixed position.

[0003] It is the object of the invention to improve the way of fixingthe bone screw, in order on the one hand to be able to applysufficiently high torques for permanent fixing of the screws and on theother hand to reduce or avoid transfer of support moments on to the boneor the vertebra during screwing down.

[0004] This object is achieved by a bone screw according to claim 1 or 2and also by a fastening tool according to claim 12.

[0005] By means of the screw connection in opposite directions bothscrew elements can be fixed at the same time. The respective supportmoment which occurs on tightening a screw element therein simultaneouslyserves as a torque supporting the screw connection of the other screwelement. In this way higher torques can be applied without a dangeroustorque being introduced into the bone of the patient.

[0006] With the tool according to the invention it is possible to applya high torque in two opposite directions, so they almost cancel oneanother out. It is further possible for the user to vary the torqueindividually for the two screw elements in a simple way.

[0007] Further advantageous configurations of the invention are thesubject of the subordinate claims.

[0008] In the following description of preferred embodiments theinvention is explained with reference to the figures.

[0009]FIG. 1 shows an exploded illustration of a first embodiment of abone screw according to the invention, illustrated partially in section.

[0010]FIG. 2 shows an illustration partially in section of the firstembodiment in the assembled state.

[0011]FIG. 3 shows an alternative embodiment of a bone screw illustratedas enlarged and partially in section.

[0012]FIG. 4 shows the side view of a first embodiment of a fasteningtool.

[0013]FIG. 5 shows the side view of the fastening tool from FIG. 4illustrated in section.

[0014]FIG. 6 shows the side view of a second embodiment of a fasteningtool.

[0015]FIG. 7 shows the side view of the fastening tool from FIG. 6illustrated in section.

[0016]FIG. 8 shows a detail C from FIG. 7 in enlarged illustration.

[0017] First Embodiment

[0018]FIG. 2 shows a bone screw in assembled state. It has an actualscrew element 1 with a screw shank 2 with a thread section 2 a and ahead 3. The head is constructed as a segment of a sphere. Coaxially tothe thread axis and on the end opposite the thread section 2 a the head3 has a recess 4 for bringing into engagement with a box spanner. Therecess 4 can also have another suitable shape, however.

[0019] The bone screw further comprises a cylindrically constructedreceiver part 5. This has at one end a first bore 7, alignedsymmetrically to the axis, the diameter of which is larger than that ofthe thread section 2 a and smaller than that of the head 3. The receiverpart 5 further has a coaxial second bore 8, which is open at the endopposite the first bore 7 and the diameter of which is large enough forthe screw element to be guided through the open end with its threadsection through the first bore 7 and with its head 3 to the floor of thesecond bore 8. Between the first and the second bore a small coaxialsection 9 is provided, which is immediately adjacent to the first bore 7and is constructed as spherical towards the open end, wherein the radiusis substantially equal to the radius of the section of the head 3 shapedas the segment of a sphere. Adjacent to the section 9 the second bore 8has a cylindrical section 17, to which an inner thread 10, extending tothe open end, adjoins. The inner thread 10 is constructed in the presentembodiment as a right-hand thread.

[0020] The receiver part 5 further has an opening in the form of aU-shaped recess 6, arranged symmetrical to the centre of the part, thefloor of which is directed towards the first bore 7 and the two sidelegs of which extend to the open end facing away from the first bore 7.An outer thread 11 is provided at the open end of the legs of theU-shaped recess 6. The outer thread 11 in the present embodiment isconstructed as a left-hand thread.

[0021] In the cylindrical section 17 a pressure element 18 isaccommodated, which is constructed in such a way that it has a sphericalindentation 19 on its side facing the head 3, the radius of which issubstantially equal to the radius of the section of the head 3 shapedlike the segment of a sphere. The outer diameter of the pressure element18 is chosen in such a way that the pressure element 18 can carry out asliding movement in the cylindrical section 17, i.e. can be displaced inthe cylindrical section 17 towards the head 3. The pressure element 18preferably has a coaxial bore 20 which enables access to the recess 4and also easier access to the pressure element 18.

[0022] A fixing screw 13 is further provided, which has an outer thread,i.e. a right-hand thread, which matches the inner thread 10.

[0023] Furthermore, a lock nut 14 is provided, with an inner threadwhich matches the outer thread 11, i.e. a left-hand thread.

[0024] In operation the screw element 1 is fed into the first bore 7from the open end of the second bore 8. With an appropriate tool, suchas, for example, a box spanner the screw element 2 can subsequently bescrewed into the bone. Then the pressure element 18 and the rod 16 areinserted in succession. The screw element 1 and the receiver part 5 inthis state are still fully swivellable with respect to one another.After this the fixing screw 13 is screwed into the inner thread 10 ofthe receiver part 5. The fixing screw 13 therein presses against the rod16, which in turn presses against the pressure element 18, which finallypresses the head 3 against the bore 7. In this way the screw element 1and the receiver part 5 are fixed both to one another and to the rod 16.

[0025] At the same time or afterwards the lock nut 14 is screwed on tothe outer thread 11 of the receiver part 5 in the screwing directionopposite the screwing direction of the fixing screw 13. The lock nut 14and the fixing screw 13 are rotated separately from one another untileach of the two parts exerts the desired holding force on the rod 16.

[0026] Owing to the thread running in the opposite direction, duringtightening of the fixing screw 13 and the lock nut 14 the directions ofrotation of the screw connections run in opposite directions. Thesubstantial advantage is that the person performing the operation canapply a stronger torque on both screw connections during tightening,without a torque being introduced into the bone or, by respectivecountering of one tool during tightening of the screw connection, thescrews being loosened again with the other tool. The stability of thescrew connections is in fact ensured or even strengthened by the mutualsupport of the counter-torques in the tightening direction of therespective screw connection.

[0027] Second Embodiment

[0028] A second embodiment according to FIG. 3 is described below.Corresponding parts are provided in each case with the same referencenumeral and the description of them in the first embodiment should bereferred to.

[0029] The screw element 1 and the receiver part 5 correspond to thefirst embodiment. Varying from the above-described embodiment, ahead-fixing screw 12 and a rod-fixing screw 13′ are provided. Thehead-fixing screw 12 is provided between the inner diameter 10 of thereceiver part 5 and the outer diameter of the rod-fixing screw 13′. Therod-fixing screw 13′ has a smaller outer diameter than the fixing screw13 of the first embodiment.

[0030] The head-fixing screw 12 has an outer thread which matches theinner thread 10 of the receiver part. In the present embodiment this isa right-hand thread. Further, the head-fixing screw 12 is provided witha coaxial bore which has an inner thread 15 which matches the outerthread of the rod-fixing screw 13′. In the present embodiment this is aleft-hand thread.

[0031] By contrast with the first embodiment the pressure element 18′ isconstructed as an elongated cylindrical element, extending from head 3to beyond the rod 16 to the lower side of the head-fixing screw 12. Thepressure element 18′ has a U-shaped recess 18 c′, open towards thesecond end 8 and extending parallel to the U-shaped recess 6 of thereceiver part 5, to receive the rod 16. The floor is constructed ascylindrical, so the rod 16 can be received. The depth of the U-shapedrecess of the pressure element 18′, seen in the direction of thecylindrical axis of the receiver part 5, is larger than the diameter ofthe rod 16 to be received, so the pressure element 18′ with lateral legs18 a′ and 18 b′ projects upwards above the rod 16. A coaxial bore, thediameter of which is smaller than the diameter of the rod 16 to bereceived, can adjoin the floor of the U-shaped recess.

[0032] In operation first, as in the first embodiment, the screw element1, inserted through the second bore 8 into bore 7, with the alreadyintegrated pressure element 18′, is pressed by screwing in thehead-fixing screw 12 on to the head 3 in such a way that it experiencestemporary rotation stabilisation. Then the rod 16 is placed into theU-shaped recesses of the receiver part and the pressure element.Subsequently the rod-fixing screw 13′ is screwed into the head-fixingscrew 12 and presses against the rod 16 into the floor of the recess ofthe pressure element 18′ and fixes the rod 16.

[0033] Finally, simultaneously both the rod-fixing screw 12 can betightened in a clockwise direction and the head-fixing screw 13′ in acounter-clockwise direction with the desired holding force.

[0034] Both embodiments have in common that in the screw elements ineach case slits, bores, etc. are provided which allow rotation with arotary tool. Advantageously the two screws are tightened simultaneously,as then the advantages of the torques in opposite directions come intoeffect.

[0035] With this embodiment the directions of rotation of the screwconnections during tightening of the rod-fixing screw 13′ and thehead-fixing screw 12 run in opposite directions. The substantialadvantage is that the person performing the operation can apply astronger torque on both screw connections during tightening without atorque being introduced into the bone or, by respective countering ofone tool during tightening of the screw connection, the screwconnections being loosened again by the other tool. The stability of thescrew connections is in fact ensured or even strengthened by the mutualsupport of the counter-torques in the respective tightening direction ofthe respective screw connection.

[0036] Otherwise, the respective directions of rotation of the threadscan of course be provided in reverse, i.e. that which was configured inthe above embodiments as a right-hand thread can be configured as aleft-hand thread and what was configured in the above configurations asa left-hand thread can be configured as a right-hand thread.

[0037] First Embodiment of a Tool

[0038] A tool, hereinafter designated as a fastening tool, is describedbelow, which is suitable for tightening the two screw connections of thetwo previously described embodiments simultaneously.

[0039]FIGS. 4 and 5 show a first embodiment of a fastening tool 21. Thefastening tool 21 has a first handle 22, from which extends a firstshank 23. The first shank is connected as fixed against rotation to thefirst handle 22. At the end of the shank 23 a first screw or engagementelement 27 is provided. The first shank 23 is surrounded by a second,hollow shank 24. One end of the second, hollow shank 24, facing thefirst handle 22, is constructed as second handle 25. At the other end ofthe second, hollow shank 24 a second screw or engagement element 26 isprovided. The second screw or engagement element 26 surrounds the firstscrew or engagement element 27 concentrically. The first shank 23 andthe second, hollow shank 24 can be rotated with respect to one another(arrow A and B in FIG. 4).

[0040] Mode of Operation

[0041] In order to fix a bone screw, for example of the secondembodiment, a first and second screw or engagement element 26, 27 isbrought into engagement with the head-fixing screw 12 and the rod-fixingscrew 13′. One hand therein grips the first handle 22, while the otherhand grips the second handle 25. By rotating the two handles in oppositedirections of rotation the first and second screw or engagement elements26, 27 are also twisted and tightened in opposite directions. It is alsopossible to perform the rotational movements of the first or the secondhandle 22, 25 differently or, for example, to rotate only one handle andmerely to fix the other handle. That means the rotational movements canbe varied depending on the torque requirement. As the torques act inopposite directions of rotation, no torque is introduced into the bonescrew, ruling out endangering the patient. The torques running inopposite directions in fact mutually cancel one another out, so notorque acts on the screw element 1. A further advantage is that by meansof the concentric arrangement of the first and second screw orengagement elements 26, 27 only a small space is needed, which is ofgreat advantage during an operation, as the operating field is alwaysonly limited.

[0042] Second Embodiment of a Fastening Tool

[0043] A fastening tool according to a second embodiment is describedbelow. Corresponding parts are provided in each case with the samereference numerals and description of them in the previous embodimentshould be referred to.

[0044] FIGS. 6 to 8 show a second embodiment of a fastening tool 21′. Asin the first embodiment, the fastening tool 21′ has a first handle 22,from which a first, central shank 23 extends. The central shank isconnected as fixed against rotation to the first handle 22. At the endof the central shank 23 a first screw or engagement element 27 isprovided.

[0045] By contrast with the first embodiment, the first shank consistsof a first shank section 23 a and a second shank section 23 b.

[0046] Gears are arranged between the first shank section 23 a and thesecond shank section 23 b. The gears are illustrated in section in FIGS.7 and 8. Hereinafter reference is made to the enlarged illustration inFIG. 8. The gears consist of a gear housing 30, composed of a firstcover 32, a substantially cylindrical sleeve component 33 and a secondcover 35. The first and the second cover 32, 35 are in each case screwedto the sleeve component 33 via countersunk screws, not shown. The firstcover 33 (sic) has a central bore 32 a, through which the first shanksection 23 a projects into the inside of the gear housing. Alignedopposite, in the second cover 35 a bore 35 a is also provided, throughwhich the second shank section 23 b projects into the inside of the gearhousing. At the ends of the first and second shank sections 23 a, 23 b,located in the inside of the gear housing 30, toothed wheels 37, 38 arearranged as fixed against rotation. Fastening of the two toothed wheelstakes place in each case via pins 39, 40, which extend cross-wise to thefirst shank through a bore, which goes through both the first and secondtoothed wheel and through the first and second shank sections. In thisway the toothed wheels are connected as fixed against rotation to therespective shank sections. In the present embodiment the toothed wheelsare constructed as bevel wheels.

[0047] In the sleeve component 33 two coaxial bores opposite one anotherare arranged cross-wise to the direction of the axis of the first andsecond shank 23, 24, through which bores a shaft 34 is pushed. Twotoothed wheels 41, 42 are held as rotatable on the shaft 34. In thisembodiment the toothed wheels are bevel wheels. The bevel wheels 41, 42roll with both the toothed wheel 37 of the first shank section 23 a andwith the toothed wheel 38 of the second shank section 23 b.

[0048] The first shank section 23 a therein acts as input shaft and thesecond shank section 23 b can act as output shaft. A torque introducedby the first shank section 23 a is transferred via the toothed wheels41, 42 to the toothed wheel 38 of the second shank section 23 b. Thedirection of rotation is therein reversed, so the second shank section23 b rotates in a direction opposite the first shank section 23 a.

[0049] Furthermore, the second, hollow shank 24 is connected as fixedagainst rotation to the cover 35 of the gear housing via at least onepin 36, in the present case via two pins. In this way during rotation ofthe gear housing the second, sleeve-shaped shank 24 is also rotated. Bymeans of the coupling of the sleeve-shaped shank 24 to the gear housingand by means of the coupling of the two toothed wheels 41, 42 to thegear housing via the shaft 34, if the second shank section 23 b is fixeda rotation of the first shank section 23 a can be transferred to thesleeve-shaped shank 24 via the gear housing 30.

[0050] As in the first embodiment, a first screw or engagement element27 is provided at the end of the second shank section 23 b of the firstshank 23. In the same way the first shank 23 is surrounded by thesleeve-shaped shank 24. The gear housing here forms a second handle 25on the end of the sleeve-shaped shank 24 facing the first handle. At theother end of the sleeve-shaped shank 24 a second screw or engagementelement 26 is provided. The second screw or engagement element 26surrounds the first screw or engagement element 27 concentrically. Thecentral shank 23 and the sleeve-shaped shank 24 can be rotated withrespect to one another.

[0051] Mode of Operation

[0052] In order, for example, to fix a bone screw of the secondembodiment, the first and the second screw or engagement element 26, 27is brought into engagement with the head-fixing screw 12 and therod-fixing screw 13′. One hand therein grips the first handle 22, whilethe other hand grips the gear housing 30, which serves as second handle.By rotating the two handles in opposite directions of rotation the firstand second screw or engagement elements 26, 27 are also rotated andtightened in opposite directions. It is also possible to perform therotational movements of the first or second handle or gear housing 22,30 differently or for example to rotate only one handle and merely tofix the other handle or the gear housing. That means the rotationalmovements can be varied depending on the torque requirement. As thetorques act in opposite directions of rotation, no torque is introducedinto the screw element 1 of the bone screw, ruling out endangering thepatient. Even if the gear housing 30 is held merely as fixed and atorque is introduced into the fastening tool only via the first handle22, the torque is transferred to both the second shank section 23 b ofthe central shank 23 and to the second, sleeve-shaped shank 24 via thegear housing 30, wherein the torques have different directions.

[0053] As in the first embodiment, a further advantage is that owing tothe concentric arrangement of the first and second screw or engagementelements 26, 27 only a small space is needed, which is of greatadvantage in an operation, as the operating field is always onlylimited.

[0054] The bone screw is not restricted to the embodiment as polyaxialscrew described here. The bone screw can in fact also be constructed asa monoaxial screw, according to which the screw head 3 and the receiverpart 5 are constructed in one piece.

1. Bone screw with a screw element (1) comprising a screw shank (2) anda screw head (3) a cylindrical receiver part (5) for receiving a rod(16) with a bore coaxial to the cylindrical axis coming from one end (8)of the cylinder, an opening (6) extending cross-wise to the cylindricalaxis for receiving the rod (16) to be connected to the bone screw, aninner thread (10) at one end (8) and a screw (13) cooperating with it,an outer thread (11) at one end (8) and a nut (14) cooperating with it,characterised in that the two threads (10, 11) are constructed asrunning in opposite directions.
 2. Bone screw with a screw element (1)comprising a screw shank (2) and a screw head (3) a cylindrical receiverpart (5) for receiving a rod (16) with a bore coaxial to the cylindricalaxis coming from one end (8) of the cylinder and an opening (6)extending cross-wise to the cylindrical axis for receiving the rod (16)to be connected to the bone screw, an inner thread (10) at one end (8)and a first screw (12) cooperating with it, which has a central bore (12a) with an inner thread (15), and a second screw (13′) cooperating withthe inner thread (15) for fixing the rod (16), characterised in that thetwo threads (10, 15) are constructed as running in opposite directions.3. Bone screw according to claim 1 or 2, characterised in that the screwhead (3) and the receiver part (5) are constructed in one piece.
 4. Bonescrew according to claim 1 or 2, characterised in that the opening (6)is constructed as a U-shaped recess open to one end (8).
 5. Bone screwaccording to claim 1, characterised by a pressure element (18) acting onthe screw head (3) of the inserted screw element (1), arranged in thereceiver part (5) above the screw head (3).
 6. Bone screw according toclaim 2, characterised by a pressure element (18′) acting on the screwhead (3) of the inserted screw element (1), arranged in the receiverpart (5) above the screw head (3).
 7. Bone screw according to thepreceding claim, characterised in that the pressure element (18′) isconstructed in one piece with the first screw (12).
 8. Bone screwaccording to the preceding claim, characterised in that the pressureelement (18′) has a recess (18 c′) for receiving the rod (16), the depthof which in the axial direction is greater than the diameter of the rod(16).
 9. Bone screw according to claim 2, further characterised by asnap ring (14′) provided in the circumferential direction on the outercircumference of one end (8) of the receiver part (5).
 10. Bone screwaccording to claim 1, characterised in that the nut (14) can be screwedon to the cylindrical receiver part (5) so far that it presses on therod (16) and fixes this by means of pressure.
 11. Bone screw accordingto claim 2, characterised in that the second screw (13′) can be screwedinto the first screw (12) so far that it presses on the rod (16) andfixes this by means of pressure.
 12. Fastening tool (21, 21′) for fixinga bone screw according to one of claims 1 to 11, with a central shank(23) constructed at one end for engagement with a screw or nut and atits other end has a first handle (22), a sleeve-shaped shank (24)surrounding the central shank (23), which is constructed at one end forengagement with a screw or nut and at its other end has a second handle(25), both shanks (23, 24) being rotatable against each other. 13.Fastening tool (21′) according to the preceding claim, characterised inthat the central shank (23) is divided in the longitudinal directioninto a first shank section (23 a) and a second shank section (23 b),between which gears with an input side connected to the first shanksection (23 a) and an output side connected to the second shank section(23 b) are arranged, wherein the gears are constructed in such a waythat the direction of rotation of a torque entering on the input side isreversible on the output side and wherein the gears have a gear housing(30) to which the sleeve-shaped shank (24) is connected as fixed againstrotation, so the input torque introduced into the first shank section(23 a) can be diverted as an oppositely directed output torque on thesecond shank section (23 b) of the central shank (23) and/or as anoutput torque directed the same way as the input torque on thesleeve-shaped shank (24).
 14. Fastening tool (21′) according to thepreceding claim, characterised in that the gear housing (30) forms thesecond handle, so that a torque introduced via the first handle (22) orthe gear housing (30) can be transferred to the central shank (23) andthe sleeve-shaped shank (24) in opposite directions in each case byfixing the first handle (22) or the gear housing (30) at desiredproportions.
 15. Fastening tool (21′) according to claim 12 or 13,characterised in that the first shank section (23 a) of the centralshank (23) is held as rotatable by a first bore (32 a) provided in thegear housing (30) and is connected at the end as fixed against rotationto a first toothed wheel (37), the second shank section (23 b) of thecentral shank (23) is held as rotatable by a second bore (35 a) providedin the gear housing (30) and is connected at the end as fixed againstrotation to a second toothed wheel (38), and at least one third toothedwheel (41, 42), which is in engagement with the first and second toothedwheel (37, 38), is held as rotatable in the gear housing (30). 16.Fastening tool (21′) according to the preceding claim, characterised inthat the third toothed wheel (41, 42) is held as rotatable on a shaft(34) standing perpendicular to the axis of the two shanks (23, 24). 17.Fastening tool (21′) according to one of the two preceding claims,characterised in that a fourth toothed wheel (42) is provided, arrangedopposite the third toothed wheel (41).
 18. Fastening tool (21′)according to the preceding claim, characterised in that the shaft (34)extends from one side of the gear housing (30) to the other side and thethird and fourth toothed wheels (41, 42) are jointly held thereon. 19.Fastening tool (21′) according to one of claims 15 to 18, characterisedin that the toothed wheels (37, 38, 41, 42) are constructed as bevelwheels.